One of the strongest predictors of how easily and well a person will learn a topic is their prior knowledge about it. The more one already knows, the easier it is to learn more. Because of this fact, students often struggle more with introductory courses, where they know little about a field, than with advanced ones, where they are adding to what they already know. There is a controversy going on in education circles about the role of knowledge versus skills. Should we teach students background knowledge, or should we focus on developing abstract skills, like reading and critical thinking? The evidence shows that prior knowledge is necessary for effective reading and critical thinking, so some educators have advocated for an emphasis on memorization of facts and information. That approach, though, is not sufficient. Cognitive research shows that useful prior knowledge is more than an accumulation of facts. Let me lay out what the research shows.